Reference
Glossary
Plain-language definitions of the terms on bulb packages, spec sheets and electricians' quotes, each linked to the chapter that teaches it.
Each definition is this manual's own short version. The chapter linked beside it gives the full explanation and the sources. Where publishers use a term differently, the chapter says so.
| Term | Meaning | Where it is explained |
|---|---|---|
| accent light | Light that picks out one object or surface on purpose | Layers, fixtures and glare |
| adaptation | The eye adjusting its sensitivity to the overall light level, which slows with age. | How light works in a room |
| airtight | Labeled for low air leakage through a recessed fixture, a separate rating from IC | Layers, fixtures and glare |
| airtight (recessed fixture) | Labeled as leaking no more than a set amount of air (2.0 cubic feet a minute under a standard test, in the model codes) between the room and the space above. | Working with an electrician |
| ambient light | Another name for general light | Layers, fixtures and glare |
| B50/L70 | The time by which half of a product population has fallen below 70 percent of initial output for any reason. | Bulbs, labels and color |
| banding | Dark bands rolling across a slow-motion video of a light, a sign of flicker the camera caught | Survey and test your rooms |
| before-and-after comparison | Reading each retest result against the first test at the same spot as a ratio or as "about the same", not as a light level | Your lighting plan |
| blocking test | Blocking the light falling on a task from in front or above; if details become easier to see, reflections are the problem | Survey and test your rooms |
| blue light hazard | In the CIE's sense, photochemical damage to the retina from staring at a very bright source; not a term for sleep effects. | Bulbs, labels and color |
| book-block test | Blocking a fixture from view with a book while looking across the room; if things become easier to see, that fixture causes glare | Survey and test your rooms |
| brief | A one-page description for an electrician of what each light should do and where it should be controlled, so that every quote answers the same request. | Working with an electrician |
| bulb trial | Buying one of each candidate bulb, using it in place for about a week at the hours the room is used, and deciding by a written return-by date to keep or return it | Your lighting plan |
| candela | The unit of luminous intensity, the light a source sends in one direction; a lumen is one candela times one steradian. | How light works in a room |
| canopy | The cover at the ceiling or wall that hides a fixture's wiring box; anything behind it is electrician work in this manual. | Working with an electrician |
| cardboard mock-up | A box or disc cut to a fixture's size and hung on string at a trial height, to judge height and size from every seat before anything is bought or hung. | Living rooms, bedrooms and stairs |
| cavern effect | A room where the floor and tables are lit while the walls and ceiling stay dark. | How light works in a room |
| circadian rhythms | The body's daily cycles, set mostly by light and dark through a master clock in the brain. | Switches, dimmers and evening light |
| coefficient of utilization | The share of the bulbs' lumens that reaches the work plane; this manual uses assumed bands for it | Layers, fixtures and glare |
| color rendering | How faithfully objects look under a light compared with a reference light of the same color temperature. | Bulbs, labels and color |
| color rendering index (CRI, Ra) | A 0 to 100 score averaging how closely eight standard color samples match their appearance under a reference light. | Bulbs, labels and color |
| confirmed | A finding checked directly that would give the same answer again, such as a wattage printed on a fixture label | Survey and test your rooms |
| control | Whatever turns a layer of light on, off, up or down: a wall switch, a lamp's own switch, a dimmer, a sensor or a remote. | Switches, dimmers and evening light |
| correlated color temperature (CCT) | The color of the light itself, given in kelvin; lower looks warmer, higher looks cooler. | Bulbs, labels and color |
| cosine correction | A meter's ability to weigh light arriving at a slant the way a flat surface receives it, quoted as f2 | Survey and test your rooms |
| damp location | In the electrical code and UL wording a fixture maker reprints, a place protected from weather but exposed to moderate moisture, such as a roofed porch. | Kitchens, bathrooms and desks |
| diffuse light | Light that leaves a frosted or fabric shade in many directions at once | Layers, fixtures and glare |
| dimmer | A control that lowers the light from a fixture or outlet as well as turning it off. | Switches, dimmers and evening light |
| direct glare | Glare from a bright source seen directly in the field of view | Survey and test your rooms |
| direct light | Light that travels straight from the source to what you are looking at. | How light works in a room |
| disability glare | Glare that impairs the vision of objects without necessarily causing discomfort (CIE). | How light works in a room |
| discomfort glare | Glare that causes discomfort without necessarily impairing vision (CIE). | How light works in a room |
| distribution | Where a fixture sends its light: direct, indirect or diffuse | Layers, fixtures and glare |
| downlight | A fixture, usually recessed in the ceiling, that aims its light down | Layers, fixtures and glare |
| driver | The electronics in an LED bulb's base that turn household current into what the LED chips need; it largely decides how the bulb dims. | Switches, dimmers and evening light |
| dry location | A place that is neither damp nor wet; a fixture rated only for dry locations carries neither the damp nor the wet marking. | Kitchens, bathrooms and desks |
| Duv | How far a light's color sits toward green (above) or pink (below) the colors of a glowing object at the same kelvin. | Bulbs, labels and color |
| efficacy | Light out per power in, in lumens per watt. | Bulbs, labels and color |
| fixture inventory | A list of every lamp and fixture in a room with its bulb, lumens, kelvin, CRI, wattage limit, enclosure and control | Survey and test your rooms |
| flicker | Rapid rise and fall in a light's output, also called temporal light modulation. | Bulbs, labels and color |
| flicker index | A 0 to 1 flicker measure that accounts for the waveform's shape but not its frequency. | Bulbs, labels and color |
| footcandle | The US unit of illuminance: one lumen spread over one square foot, equal to 10.764 lux. | How light works in a room |
| forward-phase dimmer | A phase-cut dimmer labelled TRIAC, leading-edge or INC. | Switches, dimmers and evening light |
| general light | The base layer that lights the room as a whole; also called ambient light | Layers, fixtures and glare |
| general service lamp | The federal rule's term for the ordinary household bulb covered by the efficiency standards. | Bulbs, labels and color |
| ghosting | A faint glow or flashing from an LED when its dimmer or switch is off. | Switches, dimmers and evening light |
| glare | Brightness in the field of view that is uncomfortable or makes things harder to see. | How light works in a room |
| household agreement | A few written lines, read by everyone in the home, naming which lights are on in the evening and at night, at what times, and who turns them on and off | Your lighting plan |
| IC-rated | Rated for a recessed fixture installed in contact with ceiling insulation | Layers, fixtures and glare |
| illuminance | The light falling on a surface per unit of area, measured in lux or footcandles. | How light works in a room |
| illuminance meter | The instrument that measures illuminance; usually called a light meter | Survey and test your rooms |
| indirect light | Light that reaches what you are looking at after bouncing off a ceiling, wall or other surface. | How light works in a room |
| inspection | The building department's check of finished work against the code the area has adopted. | Working with an electrician |
| integrative lighting | The CIE's term for lighting meant to serve both seeing and light's non-visual effects; "circadian" and "human-centric" are marketing terms for the same idea. | Switches, dimmers and evening light |
| inverse-square law | For a small source, the light per unit area falls with the square of the distance, so halving the distance gives about four times the light. | How light works in a room |
| kelvin | The unit used for correlated color temperature on bulb labels, shown on a 2,600 to 6,600 K scale. | Bulbs, labels and color |
| L70 | The hours until an LED's light output falls to 70 percent of its initial output. | Bulbs, labels and color |
| layer | One of the separate jobs lighting does in a room (general, task, accent, night), each with its own lights and control | Layers, fixtures and glare |
| license class | The kind of work a contractor's license covers, such as California's C-10 class for electrical work. | Working with an electrician |
| light loss factor | An allowance for light lost to bulb aging and dirt; this manual uses 0.8 | Layers, fixtures and glare |
| light reflectance value (LRV) | A paint maker's 0 to 100 figure for the percentage of light a color reflects. | How light works in a room |
| light sensitivity (photophobia) | A condition that makes a person shield their eyes from light, often to prevent pain; it has many causes. | Switches, dimmers and evening light |
| lighting binder | Every sheet in this manual saved together as one export file from the binder page, printed when someone else needs it | Your lighting plan |
| Lighting Facts label | The FTC-required panel on a bulb package giving brightness, yearly energy cost, life, light appearance and energy used. | Bulbs, labels and color |
| linear flux | The lumens an under-cabinet or strip light puts out per foot of its length, used to compare strips of different lengths. | Kitchens, bathrooms and desks |
| low vision | Sight that glasses, contacts, medicine or surgery cannot fix, in the National Institute on Aging's terms. | Living rooms, bedrooms and stairs |
| lumen | The unit of luminous flux: the total visible light a source gives off, weighted for the eye's sensitivity. | How light works in a room |
| lumen method | Working back from a target light level and floor area to the lumens the bulbs need: target × area ÷ (CU × LLF) | Layers, fixtures and glare |
| luminous flux | The total visible light leaving a source, measured in lumens. | How light works in a room |
| lux | The metric unit of illuminance: one lumen spread over one square meter. | How light works in a room |
| measurement plane | The height or surface where a target or reading is taken: the floor, a table, a fixed height, or the eye | Survey and test your rooms |
| melanopic DER | Melanopic daylight efficacy ratio: melanopic EDI divided by ordinary lux, which links a light's spectrum to its everyday reading. | Switches, dimmers and evening light |
| melanopic EDI | Melanopic equivalent daylight illuminance: the illuminance of average daylight that would stimulate the eye's melanopsin cells as much as the light in question, in lux, measured at the eye. | Switches, dimmers and evening light |
| melatonin | A hormone the body makes in the evening, under the control of the master clock, that causes sleepiness. | Switches, dimmers and evening light |
| minimum load | The lowest total wattage a dimmer is rated to control, typically around 40 watts for older wall dimmers; a few LED bulbs can fall below it. | Switches, dimmers and evening light |
| mirror test | Laying a small mirror face up on the work surface; a fixture seen in it is a source of reflected glare | Survey and test your rooms |
| mock-up | A cardboard stand-in or a held lamp at the planned size and position, used to judge a fixture before buying it | Layers, fixtures and glare |
| NEMA SSL 7A | An industry standard for compatibility between forward-phase dimmers and dimmable LED light sources. | Switches, dimmers and evening light |
| neutral wire | The conductor in a wall box that completes the circuit; many smart switches need one, and only an electrician should check for it. | Switches, dimmers and evening light |
| night light | The dimmest layer, there only to show the floor of a path at night | Layers, fixtures and glare |
| night path | The route a person walks after going to bed, usually from bed to bathroom and back, half-awake and with eyes adapted to the dark. | Living rooms, bedrooms and stairs |
| occupancy sensor | A sensor that turns lights on when it senses someone and off when the space is empty. | Switches, dimmers and evening light |
| pendant | A fixture that hangs from the ceiling on a cord, chain or rod | Layers, fixtures and glare |
| percent flicker | A flicker measure comparing the highest and lowest output in each cycle, from 0 (steady) to 100. | Bulbs, labels and color |
| permit | A building department's permission for a piece of work, usually followed by an inspection. | Working with an electrician |
| phase-cut dimmer | The common household wall dimmer, which dims by cutting off part of each cycle of the alternating current. | Switches, dimmers and evening light |
| photocell | A sensor that turns a light on when the surroundings get dark and off when they get light. | Living rooms, bedrooms and stairs |
| plan | The rows in the layers sheet for every chosen room, each naming what a layer lights, the lamp or fixture and bulb, the light and color wanted, the control, who does it and its status | Your lighting plan |
| provisional | A finding that rests on something uncertain, such as a phone reading, one evening or a flicker video that showed nothing | Survey and test your rooms |
| R9 | A separate color rendering score for a saturated red, important for skin, wood and food. | Bulbs, labels and color |
| recall check | A yearly search of the Consumer Product Safety Commission's recall list for the makers of the lamps and bulbs in the fixtures sheet | Your lighting plan |
| reflectance | The percentage of the light falling on a surface that the surface reflects. | How light works in a room |
| reflected glare | A bright source seen as a reflection in the surface you are looking at, such as a page, a screen or a counter | Survey and test your rooms |
| retest | A repeat of the first test after a change, with the same device, spots, time of day and lights, three readings averaged at each spot | Your lighting plan |
| return-by date | The last day the store accepts a trial bulb back, written in the trial sheet on the day of purchase | Your lighting plan |
| reverse-phase dimmer | A phase-cut dimmer labelled ELV or trailing-edge. | Switches, dimmers and evening light |
| Rf | TM-30's average fidelity score, 0 to 100. | Bulbs, labels and color |
| Rg | TM-30's gamut index: above 100 means colors look more saturated on average, below 100 duller. | Bulbs, labels and color |
| risk group (RG0 to RG3) | The photobiological safety classes of IEC 62471, from exempt (RG0) to high risk (RG3). | Bulbs, labels and color |
| room cavity ratio | A number describing how much wall a room has for its floor area, 5 × h × (length + width) ÷ (length × width); higher means more light lost to walls | Layers, fixtures and glare |
| room survey | A record, one row per room, of what the room is for, when it is used, its windows and surfaces, and what is wrong with its light now | Survey and test your rooms |
| safety walk | Walking a room with the safety checklist to look for wrong bulbs, cord hazards and electrical warning signs, without opening or fixing anything | Survey and test your rooms |
| sconce | A fixture mounted on a wall, aimed up, down or both | Layers, fixtures and glare |
| shielding angle | The angle, measured from the horizontal, down to which a fixture screens its light source from view (CIE) | Layers, fixtures and glare |
| slow-motion flicker check | Recording two to three seconds of slow-motion video of a light and watching for pulsing or banding; it can flag flicker but cannot show its absence | Survey and test your rooms |
| smart bulb | A bulb with a radio that dims or changes on command from an app, hub or remote, and so needs power all the time. | Switches, dimmers and evening light |
| smart switch | A wall switch or dimmer with a radio that works with ordinary bulbs and often needs a neutral wire in the box. | Switches, dimmers and evening light |
| spacing criterion | A ratio on a fixture's photometric report giving the spacing, relative to mounting height above the work surface, that yields even light from a grid; use the lowest of the values given | Layers, fixtures and glare |
| specification sheet | The maker's technical page for a bulb model, where CRI, flicker, dimming and enclosed-fixture ratings usually appear. | Bulbs, labels and color |
| spectral match | How closely a meter's sensor responds to colors the way the eye does, quoted as f1′ on a real meter's specification | Survey and test your rooms |
| switched outlet | An outlet (receptacle) controlled by a wall switch, so a lamp plugged into it works like a ceiling light; sometimes only one half of the pair is switched. | Switches, dimmers and evening light |
| task light | Light aimed at a work surface such as a page, counter, desk or mirror | Layers, fixtures and glare |
| testing-laboratory mark | A registered mark from a laboratory OSHA recognizes (UL, ETL, CSA and others) showing the product was tested and certified to a safety standard | Survey and test your rooms |
| three-wire dimmer | A dimmer that uses a neutral wire for its own power, which makes stable operation with LEDs easier. | Switches, dimmers and evening light |
| TM-30 | The Illuminating Engineering Society's newer color rendering method, reporting fidelity (Rf), gamut (Rg) and shifts by hue. | Bulbs, labels and color |
| tunable bulb | A bulb whose color temperature can be changed from an app or remote. | Switches, dimmers and evening light |
| UGR | Unified Glare Rating, a calculated discomfort-glare number for a room's fixtures, written for workplaces and rarely applicable to homes | Layers, fixtures and glare |
| under-cabinet light | A light mounted under upper cabinets to light the counter from above and in front | Layers, fixtures and glare |
| uniformity | How even the light is across a room, stated as the lowest reading against the average | Survey and test your rooms |
| unresolved | A question not yet answered, recorded with who or what will settle it | Survey and test your rooms |
| uplight | A lamp or fixture that sends most of its light upward, so it reaches the room by bouncing off the ceiling. | Living rooms, bedrooms and stairs |
| vacancy sensor | A sensor that turns lights off when the space is empty but needs a person to switch them on. | Switches, dimmers and evening light |
| wet location | A place exposed to weather or saturation, or where water can drip, splash or flow on a fixture; fixtures there are marked "Suitable for Wet Locations". | Kitchens, bathrooms and desks |